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Prevention and Treatment of Fish Diseases

Aquaculture Fisheries Division

@ Agriculture. Fisherles and Conservation Department

仰川……制帥叫…e側圳Ii1I…… 叫叫叫川…F昕跆卅i站岫s拘…liIil

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INTRODUCTION Fish diseases a何ect the survival and growth rates of fìsh under culture. Given that drug treatments are expensive, fìsh diseases invariably lead to lower harvest and higher cost. Fish farmers often suffer hefty economic losses due to fìsh diseases.

T 0 alleviate such losses, it is crucial to take precautions to prevent fìsh diseases and reduce pathogen levels in water bodies. It is also important to prevent water quality from deteriorating and to strengthen the natural resistance of the fìsh stock.

Regular monitoring of fìsh health is an e何ective way to identify disease 臼uses and appropriate treatments. One major cause of serious fìsh kill is overlooking the contagiousness of fìsh diseases and thus delaying treatment. As such, adequate care and treatment should be given to infected 白的 promptly.

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Causes of fish diseases There are three major causes of fish diseases:

>JÞ Presence of environmental pathogens >. Low resistance of the fish stock • Unsatisfactory water environment

Pathogens (e.g. bacteria, viruses, fungi and parasites) exist in all natural water bodies, yet healthy fish have adequate resistance against them. They can also adapt to 間的onable environmental changes and in turn avoid diseases due to pathogenic infection.

Environment .. 一一一一_.. Pathogens

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When the pathogen level of a water body rises sharply due to external factors, and the natural resistance of the fish stock cannot cope with the increased pathogens, the fish will become vulnerable to pathogenic infection and diseases.

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、笑 ， ‘In addition, external factors may also cause drastic changes in_ vyater quality,_ resulting i~ poor health an_d low resista r_'lce of fish stock. The risks -of pathogenic infection and fish diseases or deaths are heightened.

/4臨\4一一一. Pathogens

Environment deteriorating drastically

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To prevent and ωntrol fish diseases, we should: maintain a good culture environment and prevent the dete吋oration of water environment; and use hygienic and nutritious 自sh feed to boost resistance of the fish stock and to minimise the chance of introducing pathogens to the water body.

2.1 How can we prevent the water environment from deteriorating? 00 not over-feed - Avoid contamination caused by excessive organic matters depositing on the pond bottom/seabed.

Prompdy remove fish carcasses in fish ponds/raft net cages. Avoid contamination caused by excessive organic matters depositing on the pond bottom/seabed.

Remove fouling organisms on the raft net cages regularly, clear obstructions so that organic matters (an be removed from the fish cultu時zone by sea currents.

Read the "Good Aquaculture Practices Series 2 Environmental Management of Mariculture" and "Good Aquaculture Practices Series 3 Environmental Management of Pond Fish Culture" booklets for good practices on culture environment management.

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仙川叫……制肌叫叫e凹圳n川t…… 叫叫叫川…F昕跆岫i旭恤s鍋…hD

2.2 How to boost resistance of fish under culture? Maintain a suitable stocking density. A crowded culture environment increases the risk of disease infection and makes the fish nervous. Fish knocking against each other often get surface wounds and may develop diseases as a result.

Use a winnowing basket without knots to reduce the risk of infection caused by surface wounds.

Avoid feeding the stock with trash fish that cannot provide balanced nutrition. This type of feed lessens the natural resistance of 的sh and makes them more vulnerable to pathogenic infection.

Use dry pellet feed which is hygienic, nutritious and low in bacteria. Dry pellet feed added with vitamins and minerals can further strengthen fish immunity. Read the " Good Aquaculture Practices Series 1 Fish Feed Management" booklet for good practices on fish feed management.

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2.3 How to reduce pathogens in the water body?

Disinfect fish ponds and culture gear regularly. Refer to the methods listed in Table 1 to keep your fish farm clean.

Store the dry pellet feed properly, Keep pellets in a cool, dry and covered place to prevent massive bacterial growth.

Store trash fish properly. Pathogens may proliferate in improperly preserved trash fish. Such feed may introduce large quantities of pathogens to the water. Some pathogens can be eliminated by deep freezing. Never use trash fish that are not clean or fresh.

Table 1: Disinfection of fish farms and culture gear

|Method ..1日~

w

Fish pond bottom

Culture gear, tanksand

pipes

可 Drain and sun d叭he pond for 3 months (or shorten the t.ime by using a sterilising agent)

After draining an,d sun dryíng the pond for 3 weeks, 2 sprinkle with q_uick l.im~ (calciu'Tl oxide), ,(5.00.9 for ~e.y.ery

square metre). $un dry for another week befóre fill ing with water.

可 Di sir巾ct with electric steam gun for about 5 minutes.

Immerse in 1 :4,000 formaldehyde (i.e. 100 ml of 2 f<?rma.l~e.hyde ~，?lutio!1 for .~veryO.~ t，on ~! .wate~) for

about 1 hour, then rinse thoroughly with water.

Immerse in 1 :2,000 domestic bleach (i.e. 500 ml of bleach for 3 1 ton .ofwa叫“r about 3 hours, then neutralise with s吋ium

bicarbonate (soda) and rinse thoroughly with water.

……reatment…

Disinfect fertilised eggs and fry properly -Fertilised eggs and fry may be infected by pathogens that are present in their parents. Treat fertilised eggs with a sterilising agent and treat new fry with freshwater (for marine fish) or potassium permanganate prior to stocking.

Purchase quality fry . Fish farmers should purchase healthy fry with health certificates from reputable suppliers.

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Join the fry health inspection Fish farmers can

fry samples to the ulture, Fisheries and

Department (AFCD) for free tests on pathogens and harmful substances before placing purchase orders.

Read the "Good Aquaculture Practices Series 5 Fry Health

Management" booklet for good practices on managing

the health of fry.

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While pre個utions should be taken to prevent diseases, close monitoring of disease is equally important. It is an effective way to detect sick fish and identify the cause of disease at an early stage, so that appropriate treatment can be given and transmittable fish disease can be controlled. For this reason, the AFCD encourages 甜甜 farmers to join the Fish Health Inspection Programme and conduct a simple health check on their fish stocks every day.

3.1 AFCD's Fish Health Inspection Programme AFCD staff visit marine fish culture zones and fish ponds regularly to provide free fish health inspections and to introduce fish disease prevention practices. Services of this programme include:

〉遍Þ Full physical examination

到.. Tests for bacteria, parasites and vlruses

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〉喝. On-site demonstration of water qual ity test and fi sh disease prevention measures

>tÞ Introduction of simple fish health inspection routine

>tÞ Advice on good fish farm management

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Jτ:""\. Fish farmers are welcome to call the AFCD's Aquaculture UJ Mmgement secti州叫:訕。7089) to make i叫e仰n、』μ， bookings or obtain programme details.

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3.2 How to inspect the health of your fish stock Fish farmers should carry out a simple health inspection routine every day. To begin with, observe fish behaviour (stage one). See if the fish are reducing feed intake or showing abnormal swimming patterns. If you are certain that the abnormal behaviour is not connected with environmental facto rs, carry out a detailed health inspection (stage two). For example, check the body surface, fins and gills, and see if there are any surface parasites. If disease symptoms are detected, seek assistance from the AFCD. Our staff will visit your farm to follow up the fish disease and recommend appropriate treatment.

Stage One : Observe fish behaviour (Table 2)

• Feed intake Reduced feed intake is the first sign of many fish diseases. Fish farmers should therefore keep daily feeding records to ensure they have sufficient information to compare general intake trends.

• Abnormal Examples are fish Iying flat, rubbing against the bottom or swímmíng net cage edges, jumpi ng out of the water, circling in water pa悅凹的 or losing buoyancylbalance. AII these may be signs of disease.

Table 2: Differentiating between sick and healthy fish .tt331Uh • ___ liJi r:f.l間'lml.il..••••

Activity Swimming slowly; sluggish response Swimming actively; sharp and responsive

BodyColour Dull, dark or discoloured Bright and glossy

Body Surface White layered patches Intact

BodyShape Thin Normal size

Feed Intake Poor appetite Good appetite

Organs Different fish diseases cause Internal organs are healthy and normal damage to different organs

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Stage Two : Detailed health inspection

Check the body surface and fins - Body surface and fin wounds are obvious signs of infections. Common body symptoms of fish diseases are:

Dark body tone

Ulcer

Tumour

Haemorrhage

Tail-rot

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Loss of scales

>tÞ Check the gills - If the gills are whitened or show ulcers or with deep red spots. or there are gill flukes. excessíve mucus or obstructíve substances, the fish may be ínfected and gill functíons may be ímpaíred.

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仰川……制帥叫…e側圳Ii1I…… 叫叫叫川…F昕跆卅i站岫s拘…liIil

Diagram 1 : Fish disease monitoring f10w chart

fif('!MttI Is feed intake reducing?

Are fish exhibiting abnormal swimming patte rns

YES (..9 卸'"9目前， rubbing against the botto~ or net cage edges. jumping 。utof the water. circling hwater or losin9 buoyancylbalanc.l?

刊睡前sh are heal的y. Continue t。carry。叫 regularhealth inspection anámonitor 自sh health

NO YES

Is it related to environmental factors (..9. s.asonal chan9.s in water temperature, oil contaminàtion or red tide)?

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Common fish diseases may be caused by different pathogens, including parasites, fungi, bacteria and viruses. Sometimes diseases are not related to pathogens. Malnutrition, for example, is one reason. Fish diseases common in Hong Kong are listed in Table 3 .

Table 3 : F蚓、 diseases ζ。mmon in Hong Kong • m'l豆蓋亞Immj[，)el圖

Ectoparasitism

Endoparasitism

Fungal infection

8acterial infection

Viral infection

Non pathogen­related diseases

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Benedeniasis

clcrhytphtyoOc|ahrytoh niaslS / pntnmasls

Trichodiniasis

Chilodonellasis

Dactylogyrosis

Gyrodactylosis

Lernaeosis

Copepods

Glugea di.sease

Sanguinicolosis

Saprolegniasis

Vibriosis

Virallnfection

Unbalanced nutrition

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Marine

Marine / Freshwater

Marine / Freshwater

Marine

Marine

Marine / Freshwater

Freshwater

Marine / Freshwater

Marine

Marine / Freshwater

Marine / Freshwater

Marine / Freshwater

Marine / Freshwater

Marine / Freshwater

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The eggs or spores of most pathogens infect new hosts and transmit fish diseases through the medium of water or by direct contact. When there are environmental abnormalities, water quality deterioration, unbalanced nutrition, bodily injuries or parasitic growth which weaken natural resistance，們的 become vulnerable to pathogenic infection and diseases. The following are common fish diseases in Hong Kong and their symptoms and treatments.

4.1 Ectoparasitism

8enedeniasis

8enedenia- white and transparent, S-6mm long and 3-4mm wide.

8enedenia lay masses of e9gs. After hatching, the larvae can survive in seawater for about a day to seek new hosts.

These parasites live in the mouth, eyes, skin and fins of fish and cause bodily wounds and excessive surface mucus. The sick fish get very restless and circle continuously or rub against net surface, resulting in loss of scales a nd hence infection. The fish gradually lose appetite and eventually die of weakness and exhaustion.

Put the sick fish in freshwater and the parasitic 8enedenia will come 0何 in 1 to 2 minutes. Immerse in freshwater for 10 minutes three times every 1-2 weeks, and appl'y strong aeration.

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Cryptocaryoniasis (Seawater) / Ichthyophthiriasis (Freshwater)

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Crypfocaryon írrífans / Ichfhyophfhíríus mulfífí1lis . white, ovate or spherical and ciliated, ranging from 40 to 400 microns in length.

Juveniles can survive for more than 15 days in water after leaving a fish to seek new hosts. When they find a new host they will burrow into the fish skin and settle there.

White spots will appear on the caudal and pectoral fins and gradually extend to the body surface and gil ls. Eventually the growing white spots will form a film. The sick fish get irritable and restless. They circle continuously or rub against net surface which results in loss of scales, muscular inflammation and rot. The eyes become white and turbid and the fish gradually lose appetite, get thin and appear sluggish. Eventually gill tissues are badly damaged and the fish will die of suffocation.

Immerse in freshwater for 5-15 minutes, or immerse in solution of 1 :4,000 formaldehyde勢 or 1 :250,000 potassium permanganate* for one hour. Apply strong aeration. “For correct application of fish drugs, see Table 4

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Trichodiniasis

Tr;chod;na - about 100 microns in length with peripheral cilia.

Tr;chod;na lay hundreds or even thousands of spores which can survive in the sediment of fish ponds to wait for suitable environment for hatching. The larvae can survive for 1 to 2 days in the water to seek new hOSlS.

Symptoms are similar to those of Cryptocaryoniasis. The i nfected fish show white spOlS on the body and gills.

Similar to that for Cryρlocaryon ;,,;lan5.

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Chilodonellasis

Chllodonella . about 40 mícrons long and 30 mícrons wide.

Chl，ωdonella carry out asexual reproduction by cell division

inside fish bodies. After leaving a fish they can survive for 12 to 24

hours to seek new hosts.

Symptoms are similar to those of ichthyophthiriasis. The

infected fish have pathological changes in the body and gills.

Similar to that for ichthyophthiriasis.

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Dactylogyrosis

Daclyll句yrus ﹒ generally 0.5 mm long with 4 anterior dorsal eye spots

Daclylogyrus have full reproduction capacity at birth. They transmit fish disease by direct contact.

Daclyl句yrus cause extensive wounds on the fish body surface and increase the chance of fungal and bacterial infection. The sick fish get irritable and restless. They swim frantically to the surface or swim sideways rapidly near the bottom. Body colour darkens and movement becomes sluggish. The fish would eat less, become weak and have difficulty in breathing. Death will eventually 0 c c u r when the gills and body rot.

Use a 1:400,000 potassium permanganate solution* for extended immersion or immerse in solution of 1 :4,000 formaldehyde* for one hour and apply strong aeration. When considering to use Trichlorofon for immersion, consult veterinarian advice before treatment. 辦For correct application of fish drugs see Table 4

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Gyroda ctylosis

Gyrodactylu$ - similar to Daclylogyrus in appearance but without eye-spots.

Same as that of dactylogyrosis.

Same as those of dactylogyrosis but not always causing extensive wounds.

Same as that for dactylogyrosis.

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Lernaeosis

Lernaea - also known as anchor worms, about 1-2 cm long.

The larvae can survive for a long time in water to look for hosts.

Needle worms are parasites found on fish body surface. The sick fish will show decelerated growth due to anorexia, and develop surface wounds as they rub their bodies against objects. This increases the chance of getting other infections. Given their relatively large size needle worms can pierce through and inju陀 internal organs of young fish.

Immerse in solution of 1 :250ρ00 potassium permanganate* for 2 to 3 hours and apply strong aeration. When considering to use Trichlorofon for immersion, consult veterinarian advice before treatment. 總For correct application of fìsh drugs see Table 4.

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Copepods (commonly known as fish lice)

There are many species of fish lice. They range from several millimetres to several centimetres ín length.

Same as that of anchor worms.

Fish lice are parasites found on fish body surface. The sick fish will show decelerated growth due to anorexia, and develop surface wounds as they rub their bodies against objects. This increases the chance of getting other infections. Most lice would not cause serious harm to fish but a few species seαete toxins that make internal organs rot.

Same as that for lernaeosis.

仙川叫……制肌叫叫e凹圳n川t…… 叫叫叫川…F昕跆岫i旭恤s鍋…hD

4.2 Endoparasitism

Glugea disease

Glugea - spores are black, granular and 5-10 cm long.

Glugea spores reproduce in masses inside the fish. They are then discharged out of the bod y with urine and faeces or th roug h the body surface to look for new hosts. Dead fish with glugea disease also release large quantities of Glugea spores when they rot.

Parasitic Glugea in the large intestinal wall mucosa cause inf1ammation. The sick fish will lose appetite and gradually waste away.

None.

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Sanguin icolosis

Sanguinicola spp. 10-20 microns long.

Sanguinicolafind their way to the water through fish gills and look for snails as intermediate hosts where they reproduce in masses. When the opportunity arises they invade fish and cause damage to their circulatory system.

Parasitic Sanguinicola in the fish circulatory system will cause anaemia. They will also cause damage to gills and kidneys. There are no distinctive initial symptoms but as the disease develops, gills will show dark spots, or there may be anaemic symptoms such as bloated belly and asc忱的， inf1amed anus, upright fins and protruding eyes.

Give Praziquantel by oral administration or injection (veterinarian prescription only).

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4.3 Fungal infection

Saprolegniasis

Saprolegnia.

Saprolegnia grow in trash fish, fish feed, culture gear and benthic organic matters. Masses of free spores are released from the body surface, faeces and rotten carcass of infected fish to look for new hosts in the water. Some Saprolegnia can survive forseveral years in benthic sediments in the water. As a result, fish may get infected continuously.

Saprolegnia can burrow into the fish body from surface wounds. They can also invade the digestive tract when fish eat food containing them. They bore holes in the intestinal wall and reach internal organs through the circulatory system, reproducing and spreading across the body. Affected fish have greyish white woolly fungi on the body surface. Some Saprolegnia can cause granuloma -like lesion in int-ernal organs.

None. When saprolegniasi.s is diagnosed, all infected fish must be destroyed and no new fish are to be purchased or introduced until the fish farm is deared and thoroughly disinfected.

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4.4 8acterial infection

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Vibriosis

V伽10.

v1btio are present in water. When immunity of fish is poor or there is any surface wound, infection may occur. Fish can also get vibriosis by eating trash fish with vi伽ÎO.

Different species of Vibr的 attack different parts of the fish. Some make the body surfa役" gills and fins rot, or make the body ωm black. Othe的 attack intemal organs like the heart, liver, spleen and intestines.

Immerse in antibiotics or use them as oral drugs. 8ther way it must be prescri胎d by a veterinarian. Please note that some 惚:lrio have 從VeIOPed drug resistance to common antibiotics.

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4.5 Viral infection

Viral infection

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Lymphocystis disease virus NOdCMrUS Spring viraemia virus

Vìruses are transmitted from trash fish, btoodstock or infected fish to other fìsh. Transmission is very rapid and all the fìsh in a fìsh farm can be infected within 12-24 hours. The death rate can be 100%. Fry under one year old are particularly vulnerable. Weather or water quality changes can also lead to il1lfection.

Infection symptoms vary across di何erent viruses. The sick fìsh may swim abnormally (e.g. cirding) or show poor appetite, body wounds or darkened body colour.

None. Neither antibiotics nor other fìsh drugs have any treatment e仔'ect. However, as a preventive measure, vitamin ( can be added in the feed to boost resistance.

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4.6 Non pathogen-related diseases

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Unbalanced nutrition

Fa前yliver

Fish f，吋 r句ularfy on high fat f，便d with no essential fatty acids (such as trash fish) willω陶淵'er from toxic 陀K加1日USI叫 by peroxidation

of unsaturated fat. Appetite is poor and growth is slow. 性 mayalso

lead to bone defect or anaemia.

Improve feed qual陶" sωre fish f，僧d properly and use suitable additíves.

For details, read the "Good Aquaculture Practices Series 1 Fish Feed Management" booklet.

仰川……制帥叫…e側圳Ii1I…… 叫叫叫川…F昕跆卅i站岫s拘…liIil

one rnajor ca凶eof酬。也 fish kill is OVI叫∞king the contagiousness of 仿sh diseases and thus delaying treatment. To maximise the chance of successful treatment, infected 伯sh must be isolated for treatment immediately. If any 筒sh is found infected, isolate it immediately and give appropriate treatment or destroy it.

5.1 Rules for using fish drugs The Harmful Substances In Food Regulations (ζap 132AF) provide statue against any food containing prohibifed substances (e.g. chloramphenicol; see Schedule 2 to Cap 132AF) or containing certain substances in excessive concentration (e.g. Malachite green, Furazolidone and Tetracycline; see Schedule 1 to Cap 132AF).

Fish farmers must not use any fish drugs not prescribed by the AFζD or a registered veterinarian, or any fish drugs with unknown ingredients. When giving drugs, stick to the prescrib甜 dosage and observe the withdrawal period afterwards. In case of doubt, seek assistance from the AFζD.

The following fish drugs are not prohibited but fish farmers must adhere to the following ru les :

Hydrogen peroxide

Formaldehyde

Potassium permanganate

|lIr!'m Immerse fish in a solution of 1 :2,000 30% hydrogen peroxide (i.e. 500 ml of hydrogen peroxide for 1 ton of water) for 1 hour.

Immerse fish in a 1:10，0∞ formaldehyde solution 加 100ml of fúrmaldehyde for 1 ton of water) for 1 hour. If necessary, incr，ωse廿'le CIα'lCentratiOi'l to 1 :4，α沁0色 l∞ml of fo,malde.hyde forO.4 ωn of water). Do not use any formaldehyde solution with white sediments.

Immerse fish in 1 :250,000 potassium permanganate solution (i.e 句。f potassium permanganate for 1 ton of water) for 1-3 hours. For extended immersion, use a 1 :400，∞o potassium permanganate solution (i.e. 2.5 9 potassium permanganate for 1 ton of water) for 24 hours.

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5.2 Correct use of fish drugs Drug bath is a m司orζourse of treatment for fìsh diseases. The ζorrect way of preparing a drug bath is as follows :

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• Prepare a ∞rrect dose of fish drug, aerating equipment and a water bag seíne or large tank.

• Apply drug and ?ti! well. Put a small nurñber of fish into the drug bath fìrst and observe their reaCtion to the drug. Introduce the major _ stoc~ only when_ ~h~ reactioñ of the first batch of fish is normal.

. ωserve how the fish react n句ularlyand pump air continuously. If the nsh behave strangely, stop immersion immediately and consider other treatments.

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5.3 Safety rules for using fish drugs

〉呵.. When applying fish drugs, avoid contact wìth eyes, mou叭，skin or clothing or inhalation into lungs.

• Wear personal protection gear, such as safety goggl訟，gloves, protective dothing and mask.

• Some fish drugs are highly volatile. Maintaìn goOO ventilation at all times.

• Containers should be correctly labelled a!,d cove.red Empty containers may have residues of hazardous drugs.

〉呵.. Stay well ~Iear of ignition sour~es a~d dang~~o~s chemicals like oxides, strong acids and strong alkalis when using fish drugs

>tÞ ~hould anypart of_your. bo.dy co!,,:,e into contac_t wi!h fish drugs, rinse well with plenty of water immediately.

且.. In case of contact with eyes, rinse well with plenty of water immediately and lift the upper and lower eye lids to rinse slowly for more than S minutes. Seek medical attention right aw那

• Store fish drugs in a cool, dry and well ventilated place and away from direct sunlight.

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Technical Support Fish farmers are welcome to telephone the AFCD for free information and technical advice:

General Aquaculture Information: 2471 9142 (pond岫) / 2150 7083 (marine fish)

Fish Health and Disease : 2471 9142 仙耐側/ 2150 沌的 (閱n聞自圳

R甜Tide and Water Quality : 2150 7124

團霞，